1. Field of the Invention
The present invention relates to the art of corrosion protection by means of chemically resistant materials insulating metal articles from the effect of the atmosphere and aggressive media and, more specifically, it relates to a polymeric anticorrosion film.
The present invention is useful in simultaneous slughing and packing of metal articles in machine-building, tool manufacture, instrument making and in other industries.
2. Description of the Related Art
At present, among the most applicable means for corrosion protection of metals there should be indicated insulation of the articles to be protected with coatings and lubricants containing contact inhibitors of metal corrosion, as well as sealing of metal articles into polymeric packages with a protective atmosphere created thereinside inhibiting the occurrence of corrosion processes. The latter technique is in many cases most acceptable, since it enables corrosion protection of not only individual parts of a simple shape, but complete units and apparatus composed of them. The anticorrosion effect is attained by saturating the inner space of the package with volatile corrosion inhibitors, the source thereof being the material of the package per se, i.e. the inhibited polymeric film.
Known in the art is a two-layer polymeric anticorrosion film, wherein one of the layers is made porous so that its pores are filled with a volatile inhibitor of atmospheric corrosion, such as ammonium dicyclohexylnitrite, monoethanolamine benzoate, and the like.
The process for producing this film comprises separate extrusion of a melt of a mixture of a thermoplastic with a corrosion inhibitor and of a thermoplastic melt, followed by an adhesion joining of the two polymeric layers differing in both thickness and structure.
In the place of joining of the layers the resulting film has a distinct boundary of transition from the porous structure to the non-porous one. The porous layer has a uniform porosity and density across its thickness. For implementation of this process use is made, as a rule, of a two-slit extrusion die that necessitates additional energy consumption.
The film has sufficiently high anticorrosion properties, but its application is limited by different conditions of its exploitation. This is caused, first of all, by the use, in its manufacture, of only heat-resistant corrosion inhibitors due to high extrusion temperatures, but the range of such inhibitors is rather limited. Secondly, the films cannot be of a thickness less than 40 .mu.m at a sufficient content of the corrosion inhibitor in pores of the thermoplastic matrix.
For the manufacture of a multi-layer film with good process characteristics it is necessary that the non-porous and porous layers be made substantially from the same thermoplastic and under the same conditions of its processing. Otherwise, the multi-layer film can be separated into layers owing to different nature of the employed thermoplastics, different orientation stretching and shrinkage of polymer layers upon packing of articles thereinto, e.g. by the method of vacuum thermoforming. The separation into layers brings about deterioration of service characteristics of the film and of commercial appearance of the packages.
Known in the art is a polymeric anticorrosion film having one side thereof coated with a layer of a finely divided powder-like corrosion inhibitor. The process for the manufacture of such a film comprises extrusion of a melt of a thermoplastic, following by blow-molding thereof into a tubular article by means of compressed air saturated with a finely divided powder of a corrosion inhibitor (SU, A, 859178).
For implementation of this process use is made of an extrusion die comprising a housing, a hollow mandrel and an insert with a duct for the supply of compressed air disposed inside the mandrel coaxially thereto and projecting above its end face. The projecting part of the insert is made closed with radial holes for the supply of compressed air saturated with a fine powder of a corrosion inhibitor.
The film produced by this process does not have sufficiently high characteristics of anticorrosion properties, since a portion of the powder during its application slips down and the inhibition is non-uniform. Furthermore, the saturation of the film with a powder-like inhibitor is difficult because it does not substantially penetrate into the film but remains only on the surface.
Known is a polymeric anticorrosion film based on a thermoplastic, wherein a corrosion inhibitor is uniformly distributed over the entire volume thereof. The film is produced by extrusion of a mixture of a thermoplastic with a corrosion inhibitor, followed by blowing up the melt into a tubular article upon cooling. This process is effected in an extrusion die (SU, A, 722779) comprising a whole housing with a nozzle, a mandrel provided therein and passing through the nozzle, and having in its body at least one channel for the supply of compressed air and at least one channel for the supply of a lubricating liquid. The walls of the housing and of the nozzle form together with the mandrel walls a shaping duct passing into an annular forming slit in the zone of which in the mandrel body there is provided an O-shaped recess opened towards it. In the recess a bushing is provided made from a material permeable for the inhibitor; the inner wall of the bushing and the recess walls form a chamber communicating with the channel for the supply of the lubricating liquid. The latter liquid is supplied under pressure from the above-mentioned chamber into the forming slit upon passing the melt of a mixture of the thermoplastic with the inhibitor so that the lubricating liquid creates a uniform layer along the length of the above-mentioned slit. The use of the lubricating liquid ensures a lower coefficient of friction of the thermoplastic melt against the slit walls and enables processing of highly extended thermoplastics. The lubricating liquid provides no effect on the anticorrosion properties of the film.
This film does not possess the required anticorrosion properties, since the inhibitor is present in the thermoplastic in the encapsulated form and its liberation into the space being sealed is hindered.
Known in the art is an anticorrosion polymeric film based on a plastified polymer incorporating a corrosion inhibitor. The film structure across the thickness thereof represents a polymeric matrix with a system of pores filled with a plastifying agent or a solution thereof with a corrosion inhibitor (inhibitor liquid). The pores are uniformly distributed over the entire volume of the thermoplastic so that its density across the film thickness is not changed. The process comprises extrusion of a melt of a mixture of the thermoplastic with the plastifying agent, e.g. mineral oil, through the chaping channel of an annular die, followed by air-blowing of the melt into tube upon a continuous atomization of the corrosion inhibitor as a liquid in the cavity of the formed tubular article in an electric field.
To this end, an apparatus is used which comprises a system for the corrosion inhibitor supply, a means for the formation of an electric field inside the tubular article and an extrusion die having a hollow housing with a nozzle and a mandrel provided therein and passing through the mandrel; in the mandrel body there is provided at least one duct for the supply of compressed air and at least one duct for the supply of the inhibitor inside the formed tubular article; the walls of the housing and of the nozzle form with the mandrel walls a shaping channel passing into an annular slit. The introduction of the corrosion inhibitor into the plastified polymer is effected in the following manner.
The material coming out of the extrusion die of the extruder is in a viscous-flow state (at a temperature of about 140.degree. C.), wherefore vapors or small drops of the inhibitor are effectively absorbed by the plastifying agent, in particular mineral oil, which is a sorbent for inhibitors, and are dissolved therein. After cooling of the film and the formation of a porous matrix in the material, the inhibitor migrates to the surface together with the plastifying agent. The procedure for the manufacture of this film is rather simple, but the resulting film has insufficient anticorrosion properties for the following reasons.
First, the porous structure of the film over all its volume contributes to diffusion and, hence, to release of the inhibitor not only on the inner surface of the film and further into the sealed space of the package, but onto the external surface as well. This reduces the protective period of the film and increases a non-efficient consumption of the inhibitor. Second, the conversion of the inhibitors into a gaseous state at elevated temperatures for saturation of the inner surface of the tubular film by the inhibitor vapors in many cases lowers the effectiveness of these inhibitors due to a thermal decomposition thereof; an electrostatic atomization of liquid-phase inhibitors without heating thereof faces certain process difficulties and additional power consumption. Furthermore, the life time of the corrosion protection with such films is not long due to the impossibility of introducing the inhibitor in an amount exceeding 3-4% by mass.